Capture of Xe and Ar atoms by quantized vortices in⁴He nanodroplets

Abstract

We present a computational study, based on time-dependent Density Functional theory, of the real-time interaction and trapping of Ar and Xe atoms in superfluid He nanodroplets either pure or hosting quantized vortex lines. We investigate the phase-space trajectories of the impurities for different initial conditions and describe in detail the complex dynamics of the droplets during the capture of the impurities. We show that the interaction of the incoming atom with the vortex core induces large bending and twisting excitations of the vortex core lines, including the generation of helical Kelvin waves propagating along the vortex core. We have also calculated the stationary configurations of aHe droplet hosting a 6-vortex array whose cores are filled with Ar atoms. As observed in recent experiments, we find that doping adds substantial rigidity to the system, such that the doped vortex array remains stable, even at low values of the angular velocities where the undoped vortices would otherwise be pushed towards the droplet surface and be expelled.

Quantum-classical dynamics of the capture of neon atoms by superfluid helium nanodroplets

“…Some theoretical information has been provided on the capture of a Xe or an Ar atom by ( for Ar: v 0 = 360 m s À1 and b = 0 Å), in a TDFFT-CM study that was mostly centred on the capture of Xe and Ar atoms by HeNDs hosting quantized vortices. 47 These results are in qualitative agreement with the results obtained here for Ne, where these and other issues have been investigated in much more detail than in the previous reference: (a) most of the energy is transferred from the atom to the nanodroplet in the initial times of the collision; (b) the nanodroplet experiences large deformations; and (c) at low-moderate initial velocities of the atom it moves back and forth inside the nanodroplet. Analogous trends have also been observed in the quantum dynamics studies at zero angular momentum carried out for the Ne atom capture by ( 4 He) 1000 13 and by the Ne@( 4 He) 500 doped nanodroplet, 16 in the last case leading to the formation of the Ne 2 dimer.…”

“…In the context of the capture process when angular momentum is different from zero it is interesting to consider two initial conditions reported for the Xe capture in ref. 47: v 0 = 200 m s À1 and b = 20.3 and 22.2 Å. This velocity was selected in order to simulate the thermal conditions of the pickup chamber and the first b value corresponds to the largest impact parameter value (among the ones calculated in that reference) that leads to capture.…”

“…Regarding the collision of Xe with the HeND reported in ref. 47 for which some information is provided on the angular momentum (v 0 = 200 m s À1 and b = 22.2 Å), it should be noted that this initial condition does not lead to Xe atom capture, i.e., corresponds to a situation that differs from the main initial conditions examined here (i.e., the set of conditions leading to 47 In what refers to the vortex formation, it is worth noting that for the collision of Xe with the HeND (v 0 = 200 m s À1 ; b = 20.3 and 22.2 Å) vortex loops appear at the latest stages of the simulation, 47 in contrast to what happens in the Ne atom case. Concerning the solvation processes mentioned in Section 3.1 (zero angular momentum initial conditions), the large helium density fluctuations involved in the Rb + solvation process lead to the nucleation of quantized vortices, which can be either loop or ring vortices; while no vortices are found for the Cs + solvation, 61 and the solvation of the Ba + cation leads to the nucleation of a ring vortex.…”

“…[42][43][44] But it has not been until very recently that a few realistic theoretical studies of the pickup process and motion of the impurity inside the nanodroplet have been reported. 12,[45][46][47][48] In the present work we have theoretically investigated, using a quantum (helium)-classical (neon atom) hybrid approach, the dynamics of the pickup process of a Ne atom by a superfluid helium nanodroplet of 500 atoms, Ne + ( 4 He) N -Ne@( 4 He) N 0 + (N À N 0 ) 4 He (N = 500). To do this we have considered four representative velocities of those typical in a pickup experimental chamber (T E 300 K) and a wide set of atom impact parameters, in order to take into account the angular momentum.…”

Self-bound ultradilute Bose mixtures within local density approximation

“…The velocity field associated with the added quadrupo- lar phase, together with the angular momentum stored in the doubly-quantized vortex result in surface capillary waves, which distort the droplet surface and are responsible for the apparent rotation of the droplet as a whole [37], as shown in the bottom panels of Fig. 10.…”